The shooting occurred during an introductory geology class at the university's Cole Hall, in the campus center, around 3:15 p.m. About 163 students were registered for the class.
"The assailant began firing into the assembled class from the stage — from the front," Peters said.
"It didn't seem like he was aiming. He just raised a gun and shot immediately," said Paul Sundstrom, a student who was sitting in the class with his brother Kevin when the gunman opened fire.
Kevin Sundstrom said, "He had [a] blank stare on his face, not a frown, not a grin, like there was nobody there. I went back to find Paul. He was reloading his gun, like he's in the backyard, methodically going about it."
"I didn't think it was real, I went to [the] ground, asked a girl if it was for real. She said 'run!' I crawled and never looked back," said Kristina Balluff, another student, who was also in the hall.
"If he had [a] different gun, it would have been worse," said Kevin.
Peters described the incident as a "very brief rapid-fire assault that ended with the gunman taking his own life."
An eyewitness told ABC News' Eileen Murphy that the shooter was a white male, about 5 feet, 9 inches, wearing a black beanie and a black coat, who had an ebony shotgun. He came in through the teacher's podium area and opened fire on 100 to 120 people who were attending the class.
Grady said it appeared the gunman had acted alone.
Most of the victims were taken to Kishwaukee Community Hospital in DeKalb, though some were airlifted elsewhere, including a female with a chest injury, and two other victims with head injuries.
All of the casualties were students, including the teaching assistant, leading the class, who was a graduate student, Peters said.
Today, DeKalb County coroner Dennis J. Miller released the identities of the four victims who died in his county: Daniel Parmenter, 20, of Westchester; Catalina Garcia, 20, of Cicero; Ryanne Mace, 19, of Carpentersville; and Julianna Gehant, 32, of Meridan.
He said the gunman had "no record of police contact or prior arrest."
George Gaynor, a senior geography student, who was in Cole Hall when the shooting occurred, told the student newspaper that the shooter was "a skinny white guy with a stocking cap on," according to The Associated Press.
"Some girl got hit in the eye, a guy got hit in the leg," Gaynor told the AP just minutes after the shooting. "It was like five minutes before class ended, too."
Senior Desiree Smith left the lecture hall and ran to safety when the shooting began.
"We realized everyone was crawling out, so I started to Army crawl out of there," she said.
Student Edward Robinson told WLS-TV in Chicago that the gunman appeared to target students in one part of the lecture hall.
"It was almost like he knew who he wanted to shoot," Robinson said. "He knew who and where he wanted to be firing at."
Student Zach Seward saw teaching assistant Joe Peterson duck, "but that was all I saw," he said.
"As I was running, I just kept waiting for something to hit me in the back. I didn't know where to run, tried to decide where it's safe to be, and there isn't anywhere safe."
At 3:20 p.m., the university posted an alert on its Web site, warning students of a possible shooter.
"Get to a safe area, and take precautions until given the all clear. Avoid the King Commons and all buildings in that vicinity," the Web site posting read.
Thirty minutes later, the site confirmed the shooting and said, "several people" had been taken away by ambulance.
In December, the campus was closed for a day after a threat referred to April's Virginia Tech massacre, in which more than 30 people were killed, according to the Northern Star.
Police established a perimeter around Cole Hall and 15 to 20 emergency vehicles were on the scene in the immediate aftermath of the shooting, according to the Daily Chronicle newspaper.
More than 25,000 students are enrolled at the school, which is located 65 miles west of Chicago.